1.3.1.Compression, Encryption and Hashing (chp7)

Question 1

What is Compression?

Answer 1

• reducing the amount of storage space required by a file.

- it removes unnecessary bits to reduce overall size.

Question 2

Why is compression used?

Answer 2

• increases the number of files that can transmitted in a given time
• reduces download times
• better performance
• reduces storage requirements

Question 3

How to work out Compression Ratio

Answer 3

Compressed size/ original size

Question 4

Lossy Compression

Answer 4

• reduces file size but some of the information is lost during the process.
• The original file cannot be perfectly reconstructed.

Question 5

Lossless Compression

Answer 5

• reduces file size without losing any information.
• the original file can always be perfectly restored.
• With text documents and executable programs, use lossless because we need to be able to restore the file entirely.

Question 6

Run Length Encoding

Answer 6

(method of lossless compression)

• repeated values are removed and replaced with one occurrence followed by the number of times it should be repeated.
• relies on consecutive pieces of data being the same.
• doesn’t offer a great reduction in file size if there’s little repetition.

Question 7

Dictionary Encoding

Answer 7

• frequently occurring pieces of data replaced with an index.
• Compressed data stored alongside a dictionary which matches frequently occurring data to an index.
• Original data can be restored using the dictionary.
• its important to remember that data compressed using dictionary compression must be transferred alongside its dictionary. Without the dictionary, data cannot be used.

Question 8

Encryption

Answer 8

• the act of scrambling a piece of plain data into cipher text so that it can’t be immediately understood.
• used to keep data secure when being transmitted.

Question 9

Symmetric Encryption

Answer 9

• requires both sender and receiver to have the same key.
• the key is distributed by key exchange
• key is used for both encrypting and decrypting data
• the key must be kept secret
• if key is intercepted then communications can be intercepted and understood
• Example: Caesar Cipher

Question 10

Asymmetric Encryption

Answer 10

• two keys are used: one private (must be kept secret) and one public (can be published anywhere)
• keys are mathematically related to each other
• messages encrypted with public key can only be decrypted using private key
• Example: RSA Cipher

Question 11

Explain why asymmetric is more suited to transactions over the internet than symmetric encryption. [4]`

Answer 11

symmetric encryption would require both parties to have a copy of the key [1] This couldn’t be transmitted over the internet as it could be intercepted by anyone monitoring [1]. Asymmetric gets around this requirement as there are two different keys [1] . One key encrypts the data [1] which can be publicly distributed [1] and a different key to decrypt it [1] which is kept private [1]

Question 12

Hashing

Answer 12

• transforms a string of characters (input) into a fixed length value/ key.
• contains algorithm that converts the input into a totally different hash value
• one way process so cannot get back to the original value
• ideal fro preventing info like passwords and PINs from being read by a hacker.
• Hash functions can be used to create digital signatures